I'd say if you're going to go the average route (single 7200rpm hard drive, single cpu, recent graphics card, etc.) then a decent 350W will take care of all that. If you start adding more hungry components (dual 10,000rpm SCSI drives, single oc'ed CPU, oc'ed Ti4600 etc.) then a 400W would be recommended.
As you're going for a 1700+, I'd say get a branded 350,
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